In conventional combustion chambers for gas turbines, the main priority is to obtain a stable combustion in a wide range of operating conditions.
In order to reach this goal in the most effective way, a combustion is conventionally first carried out under conditions close to stoichiometry with part of the air coming from the compressor, and the fumes obtained are then progressively diluted with another part of the air coming from this compressor so as to lower their temperature to the thermal level allowable by the expander.
This approach however has the disadvantage of generating large amounts of nitrogen oxides (also referred to as thermal NOx) because of the very high temperatures reached in the combustion zone (flame temperatures typically ranging between 2000 and 2400° C.).
In order to comply with the new environmental regulations, gas turbine manufacturers are currently trying to develop units that can operate at full capacity, i.e. under high loads, without producing large amounts of air pollutants.
The pollutants generally produced by gas turbines during the combustion of hydrocarbons are, as mentioned above, nitrogen oxides, as well as carbon monoxide and unburned hydrocarbons. Besides, it is well-known that the oxidation of molecular nitrogen to thermal NOx in the combustion chambers of turbines greatly depends on the maximum temperature of the hot gases in the reactive zone.
The formation of nitrogen oxides can thus be represented by an increasing exponential function of the temperature. It ensues therefrom that it is possible to limit the formation of nitrogen oxides by preventing gas temperature peaks in the combustion chamber.
Several methods have been proposed for this purpose:
According to a first operating method, it has been suggested to inject water or steam into the combustion chamber to reduce the temperature peaks, which has the beneficial effect of limiting the formation of nitrogen oxides of thermal origin.
This solution is however difficult to implement compared with the results obtained because it requires sophisticated treatment of the water in order to remove all the impurities, as well as a steam generator in the case of steam injection, and it can be reasonably envisaged only for very big machines. Furthermore, lowering of the temperature considerably slows down the oxidation reaction of the hydrocarbons, which sometimes leads to a combined increase in the emission level of carbon monoxide and of unburned hydrocarbons.
Another solution consists in carrying out a multistage combustion, with a rich step and a lean step, the shift from one to the other occurring very quickly. Here again, the temperature peaks generating nitrogen oxides NOx are reduced and the rich zone allows their formation to be limited, but this solution leads to a significant production of unburned hydrocarbons.
A third solution for controlling both the temperature and the discharge of pollutants consists, prior to combustion, in mixing the air and the fuel in form of a lean mixture so as to obtain a fuel/air ratio ranging between 0.3 and 1, preferably between 0.5 and 0.8.
The air mass present in excess in the reaction zone thus absorbs part of the heat generated by the oxidation reaction of the fuel and reduces the temperature to which the reaction products are subjected. Furthermore, the cooling air requirements for adjusting the temperature at the expander inlet are markedly lower. This method thus efficiently allows to limit the production of nitrogen oxides without substantially increasing the emissions level of the other pollutants (hydrocarbons, carbon monoxide, etc.).
The main problem posed by an operation under lean mixture conditions is that premixing has to be sufficiently homogeneous and uniform to reach the desired low emissions level.
It is thus possible, for a non-uniform distribution of the fuel in the air in the combustion zone, that the presence of excess fuel in certain areas leads to the existence of hot spots, which has as a consequence the uncontrolled formation of nitrogen oxides. Similarly, in other areas of very low fuel/air ratio, local cooling prevents combustion and eventually leads to gaseous and solid unburned residues.
Besides, whereas mixing of a gaseous fuel with air occurs through the effects of molecular scattering and above all of the kinetic energy ratio between the fuel and the air, and of the spatial distribution of the injection points, mixing of a liquid fuel with air additionally requires prior spraying which conditions the quality of the mixture and therefore of the combustion.
In fact, since combustion always occurs in the gas phase, it is necessary to change the fuel into a cloud of droplets with the smallest possible diameter, evaporation of said fuel being all the faster as the diameter of the liquid droplets is small. This fuel evaporation is often carried out in two stages by means of systems provided, on the one hand, with a vaporizing chamber and, on the other hand, with liquid fuel injectors. The vaporization quality generally depends on the geometry of the vaporizing chamber.
U.S. Pat. No. 6,094,916 provides for example a device wherein mixing of the air and of the fuel is carried out under pressure by means of a fixed device equipped with radial blades generating a rotating motion of the fluid flow. A fuel injection pipe is axially positioned between each blade of the device. The fuel is injected through openings provided in the pipes with an opening angle of 60° to a radial direction of said device.
Such a layout is not suitable for injecting a fuel in liquid form because, in this case, it would have the effect of sending said fuel directly on the blades of the device, with the inevitable consequence of the formation of coke on the walls thereof, leading to considerable damage in terms of performance, lifetime of the materials of the device and emissions level of the turbine. Furthermore, it has been found by the applicant that such an injection does not allow optimum spraying and homogeneous mixing with air in case of injection of a liquid fuel.